1. Field of the Invention
The present invention relates to a manufacturing method for single crystals using the Czochralski process (hereinafter, referred to as the CZ process) and in particular to a method for manufacturing silicon single crystals by using the MCZ process in which a horizontal magnetic field is applied to a silicon melt during pulling of the single crystal.
2. Background Art
Recently, high integration and fine structurization of elements has advanced in semiconductor devices, and accordingly improvement in the quality of the silicon single crystal substrate material has been increasingly demanded. Particularly, reduction of Grown-in defects introduced during pulling of the silicon single crystal is strongly needed. Many pulling methods for reducing these defects, for example methods for pulling a silicon single crystal in an N region, have been proposed.
In addition to improving the yield of semiconductor devices and reducing manufacturing cost of IC chips, the diameter of silicon single crystal wafers (hereinafter, simply referred to as wafers) has becomes larger. Recently, wafer diameters are mainly 300 mm, a change from the conventional 200 mm diameter formerly used. The production of 300 mm-wafers has increased rapidly.
Due to the necessity to reduce the Grown-in defects and to the increasing weight of the raw materials needed for enlarging the diameter of silicon single crystals, stability of silicon melt flow during crystal growth has become more important. More specifically, to reduce the Grown-in defects of a silicon single crystal, the shape of the crystal interface and the temperature distribution in the vicinity of the crystal interface is significantly important, and stable control of the temperature distribution in the melt is therefore required.
As the diameter of the silicon single crystal increases, an increasingly large weight of raw material is required. The diameter of the quartz crucible used for CZ crystal growth has been increased from the 22 to 24 inches used for manufacturing 200 mm-diameter silicon single crystals to 32 inches for 300 mm diameter ingots. With the increasing diameter of the quartz crucible, and also the increased weight of the filling raw material, natural convection of the silicon melt becomes strong, and it becomes necessary to stably control this natural convection.
To suppress the natural convection of the silicon melt, it has recently been proposed to apply a magnetic field to the melt. In particular, a horizontal magnetic field has been proposed for effectively suppressing the natural convection of the melt. In such horizontal magnetic field processes, there have been many proposals for more effective use of conventional magnetic field intensity, with respect to the magnetic field distribution in the silicon melt.
For example, Japanese Laid-Open (kokai) Patent Application H08-231294 discloses that change in oxygen concentration in the crystal growth direction becomes large when the longitudinal (crystal axis direction) distance between the silicon melt surface and a horizontal magnetic flux axis is larger than 5 cm, and accordingly the longitudinal distance between the silicon melt surface and the horizontal magnetic flux axis is fixed to within 5 cm to control the oxygen concentration distribution in the crystal growth direction.
Moreover, Japanese Laid-Open (kokai) Patent Application H08-333191 proposes that to enhance the uniformity of the applied magnetic field intensity and to improve the convection suppressing effect in the silicon melt over the entire crucible, the relative positions of an electromagnet and the crucible in the vertical direction is set so that the central axis of the coil coincides with the depthwise central part of the melt in the crucible or below this central part.
Moreover, Japanese Laid-Open (kokai) Patent Application 2004-182560 proposes that, in the use of a curved, saddle-shape coil, to prevent the phenomenon of rapid increase in crystal diameter, to perform stable pulling, and to avoid deterioration of the inplane oxygen concentration, the coil axis is positioned away from the raw material melt surface by 100 mm or more in depth.
Furthermore, Japanese Laid-Open (kokai) Patent Application 2005-298223 discloses reducing Grown-in defects and obtaining a high-quality crystal by positioning the magnetic field center in the range of 100 mm to 600 mm below the melt surface.
As described above, many different embodiments have been proposed for the positional relationship of the horizontal magnetic field in the silicon melt for various purposes, such as for enhancing stability of the oxygen concentration distribution or uniformity of the magnetic field strength so as to suppress the convection of the silicon melt and to reduce Grown-in defects.
When a silicon single crystal is pulled from a molten silicon in a quartz crucible, it is important to keep the crystal thermal history in the vicinity of the crystal interface constant for maintaining uniformity of both oxygen concentration and uniformity of Grown-in defects in the longitudinal direction. Therefore, in conventional CZ manufacturing equipment, the quartz crucible containing the molten silicon is raised in accordance with the weight of the pulled single crystal, so that a constant distance is maintained between an Ar baffle plate disposed at a fixed position and the molten silicon surface, in other words, the liquid surface position of the silicon melt with respect to the Ar baffle plate is maintained at the same position during the entire pulling processes.
In such a manufacturing method, the relation between the molten silicon and the center position of a horizontal magnetic field is as shown in FIG. 4. More specifically, in an initial state in which a sufficient amount of a silicon melt 43 is housed in a crucible 41, as shown in FIG. 4A, the magnetic field center position 45 of the applied magnetic field which is away from the liquid surface 44 of the silicon melt 43 in the crucible 41 by a distance H is positioned between the liquid surface 44 and a lowest part 46 of the silicon melt 43. In this state, pulling of the silicon single crystal is started, and raising of the crucible 41 is performed in accordance with the amount of the silicon single crystal 47 which has been pulled. When pulling of the silicon single crystal 47 has advanced, and the rise of the crucible 41 becomes K, as shown in FIG. 4B, the lowest part 46 of the silicon melt 43 reaches the magnetic field center position 45. When the pulling has further advanced, as shown in FIG. 4C, the lowest part 46 of the silicon melt 43 is positioned above the magnetic field center position 45.
In the manufacturing method shown in FIG. 4, the effects of the magnetic field exerted on the silicon melt is thus largely different between the states illustrated in FIGS. 4A, 4B, and 4C.
When the relation between the liquid surface 44 of the silicon melt 43 and the magnetic field center position 45 passes through the state of FIG. 4B and attains the state of FIG. 4C in which the symmetrical property in the vertical direction about the magnetic field center position 45 of the silicon melt 43 is lost, numerous problems influence the silicon melt 43 and the silicon single crystal 47 during pulling, as described hereinafter.
More specifically, when the magnetic field center position 45 of the applied magnetic field is lower than the lowest part 46 of the silicon melt 43 in the crucible 41 (state of FIG. 4C), the temperature of the silicon melt is periodically changed. As a result, the diameter of the pulled silicon single crystal is periodically changed in the longitudinal direction, and can become smaller than the predetermined diameter, which causes yield reduction in the manufacture of the silicon single crystal ingots. To prevent this reduction in diameter below specification, it is conceivable to increase the target diameter of the manufactured silicon single crystal ingot in advance. However, increasing the target diameter also reduces yield due to the increased diameter elsewhere, and thus this method cannot prevent reduced yield.
Moreover, in the state of FIG. 4C, the rotational speed of the crucible 41 may be decreased to lower oxygen concentration, and the magnetic field may be enhanced to further suppress natural convection of the silicon melt. However, under such crystal pulling conditions, as a unique phenomenon associated with use of a horizontal magnetic field, part of the molten silicon surface solidifies in low-temperature regions of the silicon melt surface, the solidified silicon ultimately contacts the single crystal during pulling, and causes dislocations in the pulled crystal. Therefore, all the crystal pulling conditions described above cause yield reduction in silicon single crystal manufacture.
Moreover, when a magnetic center position 55 of an applied magnetic field is at a deep position of a silicon melt 53 in an initial state like Patent Document JP2004-182560 or JP2005-298233 (see FIG. 5A), the state of FIG. 4C is already attained as shown in FIG. 5B before completion of the pulling of the straight body (“constant diameter”) part of the silicon single crystal 57, that is, before tail-in. From the middle of the constant diameter pull, after the lowest part 56 of the silicon melt 53 reaches the magnetic field center position 55 (see FIG. 4B), and until the beginning of tail-in (FIG. 5B), the crystal diameter is cyclically changed, part of the surface of the molten silicon is solidified and brought into contact with the crystal during pulling, causing dislocations, with a concomitantly large decrease in yield.
When the magnetic field center position 65 of an applied magnetic field is set at a shallow position, for example, a position that is within 5 cm from the liquid surface 64 of a silicon melt 63 as in H08-231294 (see FIG. 6A), generally, a depth L from the liquid surface 64 of the silicon melt 63 remaining in the crucible 61 at the point of tail-in is larger than 5 cm, and the crystal pulling process is completed without generating the above described states of FIG. 4B to FIG. 4C, as shown in FIG. 6B. Therefore, during the crystal pulling process, the temperature of the silicon melt is not periodically changed, and no reduction in yield due to periodical change of crystal diameter below specification in the longitudinal direction is experienced.
However, when the magnetic field center position is too close to the liquid surface of the silicon melt like the case of FIG. 6A, the phenomenon of unstable silicon melt flow appears. Particularly when the weight of the silicon melt is large, this phenomenon becomes serious. At the beginning of the silicon single crystal pulling step, in other words, in the necking step, the diameter increasing (cone) step, and the constant diameter step, the temperature of the silicon melt becomes unstable, and crystal dislocations are generated. As a result, it has been necessary to repeat the process of melting the dislocated crystal again and performing pulling. Therefore, productivity of a silicon ingot is reduced significantly. Also in the case in which the distance between the liquid surface of the silicon melt and the magnetic center position is large, the convection of the silicon melt also becomes unstable, and a similar problem occurs.
On the other hand, to enhance the uniformity of the magnetic field strength of the applied magnetic field, as described in H08-333191, it is proposed that the relative positions of the electromagnet and the crucible in the vertical direction is set so that the center axis of the coil goes through the depthwise center part of the melt in the crucible or below of the center part. According to this method, when the weight of the silicon melt is large, the central axis of the coil is below the depthwise center of the silicon melt, and the central axis of the coil is never below the lowest part of the silicon melt even at the end of the pulling step. Therefore, such a method can be conceived to be an effective method for solving the above described problems.
However, in such method, the remaining amount of the silicon melt is reduced as pulling of the crystal advances; accordingly, the distance from the liquid surface of the silicon melt in the crucible to the coil center axis is gradually reduced, and the magnetic field distribution applied to the molten silicon in the vicinity of the part immediately below the crystal interface is changed through all the pulling steps of the crystal. Therefore, stability of the magnetic field strength applied to the molten silicon cannot be obtained.
As described above, in the conventional manufacturing methods of the silicon single crystal, to achieve stabilization of the flow of the molten silicon, particularly stabilization of the molten silicon flow in the vicinity of the region immediately below the crystal interface which directly affects stabilization of silicon single crystal growth, the intensity distribution of the magnetic field applied to the crystal in this region has been required to be controlled so as always to be constant.